Where will the Chinese space station fall? Good estimates will come “only in the hours just before re-entry.” | Miles O'Brien Productions

Where will the Chinese space station fall? Good estimates will come “only in the hours just before re-entry.”

By now, you’ve probably heard about the Chinese space station, Tiangong-1, which is spiralling in an uncontrolled descent into the atmosphere. Two years ago, the Chinese announced that they had stopped communicating with the station and were monitoring its descent. Now, it is set to finally plunge into the atmosphere likely sometime this weekend.

WATCH: Animation by The Aerospace Corp. explaining the Tiangong-1 re-entry (school bus not included).

To talk through the situation, I recently sat down with Marco Langbroek, a Dutch satellite tracker who has been following the fall of Tiangong-1. (You may recognize him from our North Korea Nukes series for the PBS NewsHour.) Below is the transcript of our discussion, which has been edited for clarity.

Marco Langbroek. Photo: Twitter. | Miles O'Brien Productions
Marco Langbroek. Photo: Twitter.



Miles O’Brien: Let’s do a little bit of background. What do we know about this particular space station? The Chinese don’t share an awful lot, do they? What do we know, though?

Marco Langbroek: What we do know is that over a year it’s gone down and the Chinese have at some point lost telemetry contact with the space station. It’s not entirely clear how much control they still have over the station, but it’s coming down. Basically within now to two more weeks, it will be toast.

Miles O’Brien: So, the station itself–again, the Chinese don’t share a lot–the fact that we don’t even know for sure whether they have telemetry or not is a little bit unusual. Skylab or Mir, a lot of information was shared. Does this make it more difficult for the rest of the world to understand what’s happening?

Marco Langbroek: Well, this is not the first uncontrolled re-entry. In fact, Skylab at the time was also an uncontrolled re-entry.

We do know a few things. We know how big it is. We know how much its weighs. It’s 8 and a half tons, which is a lot. It’s about 10 and a half meters long, three and a half meters wide–and that’s excluding solar panels. That’s big enough and heavy enough to know that if it re-enters into the atmosphere there will be some parts that survive, and some parts will come down to ground level.

Skylab was a huge space station, the first huge space station basically. And at one point it came down as well, uncontrolled. And in the end, pieces of it ended up in Australia and the Australian government sent in an invoice for NASA. [They didn’t pay up]

Miles O’Brien: But to go back to that era, people where a little bit freaked out about Skylab coming down as I recall?

Marco Langbroek: Yeah, that was actually the first time that a really big object came down. People were quite scared, and I think also in this case of the Chinese space station, but there’s not really reason to be very scared. Yes, it’s big, yes, parts will survive but most likely they will end up in the ocean.

Miles O’Brien: Relative to Skylab & Mir, how big is this space station?

Marco Langbroek: It’s [9 times] smaller than Skylab and a lot smaller than Mir was, but it’s still big, yeah. It’s one of the biggest objects to re-enter in the past few years.

Miles O’Brien: Let’s talk briefly about how this is different from the re-entry of Mir.

Marco Langbroek: Mir was a controlled re-entry. With Mir, they sent the orbit burn command at specified moments, so they could very closely determine where it would come down and make sure it came down over the Pacific Ocean. So it wouldn’t harm anybody, just dumped in the ocean basically. And that’s the difference with this re-entry. This re-entry is, as far as we know, uncontrolled. You never know, maybe the Chinese got a surprise, but it looks to be an uncontrolled re-entry.

Miles O’Brien: Generally speaking, most of it will burn up. But there are specific pieces that will make it to the ground?

Marco Langbroek: Yeah, certainly, and especially parts of the rocket engines. There are pressure spheres and tanks. Those especially have a tendency to survive.

Miles O’Brien: A thing that most people don’t realize, these re-entries happen a lot, don’t they?

Marco Langbroek: Multiple each month: rocket stages, satellites and parts of rocket stages, it happens a lot. A lot of them are uncontrolled, actually.

Miles O’Brien: As I recall, I can’t remember the name of it, but there was a Russian satellite that had some sort of radioactive aspect to it that ended up, pieces of it landed in the northern territories of Canada. Am I recalling that correctly? Tell me about that one.

Marco Langbroek: Kosmos 954. That Kosmos satellite was so called RORSAT, and it was a very large radar satellite. It had a nuclear reactor on board and it came down in 1978 off of Northern Canada. It basically contaminated small parts of Canada with radioactive isotopes.

Miles O’Brien: So that’s a notable re-entry story. There’s also the famous story of the one individual on the planet who has been struck by a piece of a rocket. I believe her name is Lottie Williams, is that right?

Marco Langbroek: It was a very small piece of a rocket stage and apparently she was hit. And she survived.

Miles O’Brien: She said it was like being tapped on the shoulder. I think she ended up with a piece that wasn’t particularly heavy.

Marco Langbroek: No, it was very lightweight and a large surface, so basically once it’s through the atmosphere it wiggles down like a leaf from a tree basically.

Miles O’Brien: I remember post-Columbia. Some of the pieces of paper survive, which is really interesting when you think about it, right? Because they were kind of aerodynamically slowed down, right?

Marco Langbroek: Very light objects with a long surface, they slow down very quickly and it basically means that they have a bigger chance of survival, yeah.

Miles O’Brien: So the light stuff has a good chance of surviving. And do we know what percentage we could expect to actually make it to the surface, whether it’s the water or land?

Marco Langbroek: Difficult to estimate. To really estimate that, you need details of how that space station is constructed, which I don’t have, but anything from 10% to 20% is I think a good estimate.

Miles O’Brien: So how is this different from those typical pieces of rocket parts that are just a consequence of normal space operations?

Marco Langbroek: It is difficult because this is a quite a heavy and large object. I mean, rocket stages can be quite large, but they’re usually just a hollow shell, basically, and this is a much more massive object, and that means probably a lot more it will survive in normal rocket space re-entry.

Miles O’Brien: Okay, so now walk us through what’s happening in orbit right now.

Marco Langbroek: Many aspects of the earth’s atmosphere, extend much further in space than most people think.

So, even though the atmosphere is very low density at that altitude, it’s still present and spacecraft at altitudes below 500 kilometers really know that.

The friction with that very low density atmosphere is enough to, over time, slow it down and make it go towards the earth so it’s basically falling down.

Miles O’Brien: Tell me what you’ve been observing.

Marco Langbroek: I’m not tracking it myself actually because this space station is not visible from the Netherlands. We’re too high in latitude, but we’re looking at the orbit data and what we see is that over the past year, that it’s considerably come down from over 400 kilometers and currently it’s below 250 kilometers. That altitude is very low for a satellite and it’s coming down by multiple kilometers per day at this moment.

This is tracked by the huge military tracking stations.

Miles O’Brien: Okay, and that data is released to the world so everybody knows what’s going on?

Marco Langbroek: Yeah, and you can analyze it.

Miles O’Brien: So, that is derived by what radar telemetry or how do they come up with that or is that optical?

Marco Langbroek: It’s a combination of radar observations and then optical tracking, tracking with telescopes basically.

Miles O’Brien: So, the likelihood is that no one will see a single piece of this. Is that accurate to say?

Marco Langbroek: If you look at the area where it can come down, the majority of that area is ocean. So it’s most likely that it will end up somewhere in the Pacific or the Atlantic Ocean or the Indian Ocean. But there are a few landmasses in that area as well, the whole of Africa, Australia, New Zealand, the southern parts of Europe and Asia, South America, and United States are all basically areas where it could come down in principle.

Miles O’Brien: So, how worried should people be?

Marco Langbroek: Not too worried. The chances that it ends up in the ocean are bigger than that it ends up somewhere on land, and then a lot of that land surface is desert and remote area. Very little people live there. So, the chances that it will end up in a highly populated area are not that big.

Miles O’Brien: So, this does raise an important point that when spacefaring nations launch satellites, there is certainly–while there’s no laws–there is a moral responsibility to come up with a way to safely return it, right?

Marco Langbroek: Yes, most launching organizations nowadays are trying to incorporate the possibility of de-orbit into their payloads, usually not in the rocket stage. Of course SpaceX is experimenting with return stages, which is a good thing, I think. But for the large payloads, it’s seen as responsible to make sure that if it is in a low orbit that they can de-orbit it at the moment that they wish.

Miles O’Brien: Is it accurate to say the Chinese did not plan for this in this case?

Marco Langbroek: I think that’s accurate. Apparently they didn’t–China is a relatively new country when it comes to space. They are quite quickly turning into a major launching nation. But with these kind of large projects, yeah, they still have to learn a lot, I think.

Miles O’Brien: Well, it wasn’t too many years ago that they used an anti-satellite device to destroy one of their own satellites for practice, creating a huge problem of debris in lower earth orbit. What does that say about the Chinese responsibility in space?

Marco Langbroek: Unequivocally seen by everybody as highly irresponsible at the time. I think the Chinese learned from that, the diplomatic repercussions of it were that big that they won’t do that a second time I think. There’s still a lot of debris from that satellite test in orbit and at low orbit, at the orbital altitude of, for example, the International Space Station. That kind of endeavor, you should not do that.

Miles O’Brien: So, the biggest thing up there in low earth orbit right now of course is the International Space Station. Based on what you’ve seen, is there a good solid plan for bringing it down safely when the time comes?

Marco Langbroek: I’m not so sure actually. What I do know is that the Russian segment has its own propulsion system. So, I presume they can bring it down when they want, but the European and American segments don’t have those propulsion systems. So, if you want to bring those down, we’d need to keep them attached to the Russian part, or send something up and couple to it to bring it down.

Miles O’Brien: So this is kind of a real world, 21st century version of Chicken Little I guess, right? The sky is falling?

Marco Langbroek: Tthe sky is falling, but we should remember that the sky is falling each month. Basically each month, somewhere on this earth, something from space comes down. So we should not make too big a deal of it, I think.

Miles O’Brien: Unless you’re Lottie Williams in Oklahoma?

Marco Langbroek: There’s always a possibility. But then you can also walk down the street and get struck by a bus. I think the chances that that happens are bigger than that you get struck by a space station.

Miles O’Brien: Tell us about the difficulties of predictions. If you don’t have the ability to bring it in in a controlled fashioned as they did in the case of Mir, why is it so hard to predict exactly when? Because I know it’s plus or minus six days I believe or something like that? Walk us through.

Marco Langbroek: Tthe uncertainties are in the days. In the hours directly before the real re-entry, we can narrow it down to a couple tens of minutes. And there are various factors that play a part in that.

The density of the atmosphere is not constant. It also depends on whether the space station spends a significant part of its orbit in the nighttime part of the earth or the daytime part of the earth.

There’s also the fact that the space station might not be stable in its orientation and depending on what part of the space station faces forward. If a very, very large part points forward, it will experience more drag than if a more narrow end of the space station points forward. So those are all factors that play a role and they are actually very difficult to model even with computers.

At this moment, we can make estimates within a certainty of a few days and then just before re-entry, the last two days or something like that, it will become less than a day. Only in the hours just before re-entry the observer team will know on the order of a few tens of minutes. But these objects, they move with the speed of seven kilometers per second, so even if you have 10 minutes of uncertainty, you have thousands of kilometers that can make a difference between going down over the ocean or going down over Spain to say something. So, until the very end, it’s very difficult to predict where it will come down.

Miles O’Brien: But statistically, I think the planet is 75% to 80% water, right? So, statistically, you’re better off buying a lottery ticket, right? You got a better chance of winning than you do getting hit by one of these pieces, don’t you?

Marco Langbroek: The chances that you’re going to get hit are small. Human population still tends to be concentrated in rather dense spots, in cities.

And if you look at the surface of that it’s basically is not much, surface is low. So the amount of earth surface density is partially inhabited or is ocean is much, much larger than the really densely populated areas.

Miles O’Brien: The inclination is, is it 49 degrees or 42 degrees? I can’t remember what is it.

Marco Langbroek: 42.8.

Miles O’Brien: 42.8. So anywhere from a latitude of 42.8 North and 42.8 South is fair game, right?

Marco Langbroek: Anywhere in that area it can come down. That includes the whole of Africa, it includes the United States, South America, Australia, and some part of Europe and Asia. I myself am safe. We are too high inland at 51 North, but Southern Europe, Spain, for example in principle, are in the danger zone.

Miles O’Brien: So, we should all come to the Netherlands and ride this out with you?

Marco Langbroek: Yeah!

Banner image credit: ESA.

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